Photoelectric detectors are devices in which radiant energy incident thereon produces electrical effects. Detectors of this kind are used in a wide variety of applications. One known detector is referred to as a charge-couple device (CCD). Essentially, charge couple devices convert incident spectral energy to video signals which are used subsequently in, for example, electronic imaging devices.
For well known reasons, it is desirable to have the spectral sensitivities of such photodetectors match generally that of the human visual response. However, there are a number of factors causing differences between these sensitivities. A rather common cause is the nature of the material used for the photodetector. For example, charge-couple devices (CCD.sup.s) of the silicon type have relative spectral responses more sensitive to red and infrared wavelengths and tend to be less responsive to blue light. Accordingly, the blue content of the images generated from such video signals tend to be noisy (i.e. there is a relatively high signal to noise ratio). This is, of course, undesirable.
Another factor affecting the response of the photodetectors is the nature of the relative spectral composition of the light incident thereon. For instance, in familiar artificial illumination conditions involving tungsten-type light, there exists a relatively small blue spectral component. Hence, the resulting blue component of the output signal generated by the photodetector will be relatively weak. Such weakness is, of course, compounded if the photodetector itself, tends to be of the silicon type, which as noted is relatively less sensitive to blue light.
It is well known in the field that one can alter the spectral sensitivity of photodetectors so as to compensate for inherent biases in sensitivity of the photodetectors themselves or for certain scene lighting situations.
It is common practice to place a filter element or combination of filter elements in the optical path before the photodetector in order to correct the spectral response. By change or correction to the spectral response, it is meant that the spectral composition of the radiant energy incident on the photodetector is altered so as to conform it with a predetermined standard or reference, for instance, the sensitivity of the human eye or of a particular CCD or photographic film.
Commonly assigned U.S. Pat. No. 3,903,413 describes use of an optical filter having spectral absorption characteristics that change or correct the spectral sensitivity of a silicon type photodetector. In this manner, the photodetector was made less sensitive to red and near infrared spectral wavelengths. Therefore, the output of the photodetector is relatively more sensitive to blue spectral wavelengths incident thereon.
With reference again to the fact that the charge-couple devices are relatively insensitive to blue wavelengths, it is known to use photodiodes that are doped suitably so as to be more sensitive to blue light. Alternatively, it has been proposed to boost the gain of the blue component of the signal from the charge couple device to compensate for such relative insensitivity.
There are other approaches for modifying the spectral composition of scene light incident on, for example, a silicon type photodetector. The following commonly assigned patents are examples of some of these approaches: U.S. Pat. Nos. 4,351,593; 4,325,616; 4,315,675; 4,358,186; and 4,428,654. All of the above patents, with the exception of the last one, are generally directed to alternating between different spectral wavelengths during an exposure. This is achieved essentially by controlling scene light passing through a photocell lens that cooperates with different spectrally filtered zones. In U.S. Pat. No. 4,428,654 there is disclosed a spectral filter stationarily interposed between a photocell lens and detector. The filter includes different spectral segments which correspondingly direct different proportionalities of wavelenghts simultaneously to the photodetector. In this manner scene light from different scene zones are treated differently. However, in this situation, there is no uniform distribution of the wavelengths over the entire photodetector, but rather the formation of distinct spectral zones on the photodetector itself.
Another known approach for modifying spectral composition of light incident on the photodetector utilizes bands of red, green and blue filters at the aperture stop of an imaging lens. Such a filter arrangement has been used for converting black and white images into color images. In such an approach a lenticular plate is used to dissect the light into red, green and blue stripes. This approach solves a different problem than that which the present invention solves, and again distinct spectral zones are formed on the photodetector itself.